Within the medical field, ultrasonic systems are used for various imaging and/or treatment purposes. While many of the medical techniques may be non-invasive, i.e. only the ultrasound enters the body of interest, certain procedures require the introduction of a medical device into a person. Non-invasive ultrasonic imaging and treatment may be inadequate as a result of inaccessibility of tissue to an acoustic beam. A second concern relates to frequency. Higher ultrasonic frequencies are desirable, since short wavelengths enhance resolution. However, higher frequencies attenuate more rapidly, limiting the depth of penetration. At least to some degree, intracorporeal ultrasonic devices overcome these limitations.
One type of intracorporeal ultrasonic device is described in U.S. Pat. No. 4,870,953 to DonMicheal et al. The patent describes an apparatus and method for utilizing ultrasonic energy for the destruction of plaque or a blood clot in the blood-vessel. A hollow catheter is introduced into the blood vessel. An elongated, solid, flexible probe extends through the catheter. At a distal end of the probe is a blunt probe tip that is configured to minimize the likelihood that the probe tip will perforate the blood vessel. At the opposite, proximal end of the probe is a source of ultrasonic energy. Conventionally, the energy source abuts the end of the probe, so that the ultrasound is axially introduced into the probe for transmission along the length of the probe.
A similar device is described in U.S. Pat. No. 5,304,115 to Pflueger et al. The probe is referred to as an ultrasound transmission member and is formed of one or more superelastic metal alloys, such as a nickel-titanium alloy having a content of approximately 50.8 atomic percent nickel. Again, an ultrasonic energy source is coupled at a proximal end of the transmission member, while the distal end has a bulbous member. The bulbous distal end is caused to vibrate by operation of the energy source. The vibrations are designed to fracture arterial lesions and the like. Such angioplasty techniques have been successfully utilized.
Catheter-based imaging techniques are also known. Ultrasonic transducers may be located at the catheter end that is introduced into the body or at the catheter end that remains exterior to the body. Acoustic waves are directed at tissue of interest, whereafter reflected energy is detected. The reflections occur as a result of changes in acoustic impedance, so that time delays in receiving the reflections or phase changes of the reflections can be used to form images of organs or other tissue.
As previously noted, increases in frequency potentially enhance ultrasound image resolution. Higher frequencies are also preferred for angioplasty devices for pulverizing arterial lesions. However, conventionally ultrasound is at approximately 20 KHz for such applications, since ultrasound at higher frequency quickly attenuates when propagated through a transmission member such as the type described in DonMicheal et al. and Pflueger et al.
What is needed are an apparatus and a method for coupling ultrasonic energy to a longitudinal transmission line such that sufficient power is obtained for operation at high ultrasound frequencies.